Apparatus and method for detecting epilepsy seizure

ABSTRACT

An apparatus for detecting an epilepsy seizure, includes: a graph generating unit configured to generate an orthogonal graph where values of heart beat intervals are sequentially plotted while a heart beat interval at an arbitrary timing is set as a first coordinate and a next heart beat interval is set as a second coordinate; and a seizure detecting unit configured to detect the epilepsy seizure based on: a change of a distribution of a group of the plotted values in a first direction perpendicular to a straight line passing an origin and a point where the first and second coordinates are equal to each other; and a change of a distribution of the group of the plotted values in a second direction parallel to the straight line.

This application claims the priority benefit under 35 U.S.C. §119 ofJapanese Patent Application No. 2012-0067709 (filed on Mar. 23, 2012),which is hereby incorporated in its entirety by reference.

BACKGROUND OF THE INVENTION

The present invention relates to an apparatus and method for detectingan epilepsy seizure.

A brain wave is obtained by measuring a change in potential differencecaused by a weak electrical activity of the brain by means of scalpelectrodes or intracranial electrodes or the like, amplifying a measuredsignal, and expressing the amplified signal as a waveform. An epilepsyseizure means various signs and symptoms derived from excess dischargeof neurons of the cerebral cortex.

An epilepsy seizure is detected as a sudden abnormal wave of a brainwave. Usually, a brain wave obtained by using scalp electrodes is usedin the detection. However, a brain wave obtained by this method issusceptible to various kinds of noises. In an epilepsy seizure, a noisewhich largely affects a brain wave is an electromyogram which is inducedwith myotonia. An epilepsy seizure often involves a motor seizure and abody motion. Therefore, it often occurs that brain wave signals arecovered by an electromyogram and an epilepsy seizure cannot be detectedfrom a brain wave. Furthermore, it is often that an epilepsy seizurelocalized to the deep brain such as the medial-temporal lobe cannot bedetected from a brain wave obtained by using scalp electrodes.

By contrast, a brain wave obtained by using intracranial electrodes isnot affected by an electromyogram as compared to a brain wave obtainedby using scalp electrodes, and therefore it is possible to record anelectrical activity in the deep brain. However, an installation ofintracranial electrodes requires a neurosurgical procedure, and hence ishighly invasive. Moreover, there is a risk for complications. Therefore,such an installation imposes a heavy burden on the patient.

As a related-art method of detecting an epilepsy seizure without using abrain wave, there is a method in which the heart beat rate is measuredby seconds, and an epilepsy seizure is detected by an appearance of abradycardia following a tachycardia (refer to JP-T-2009-519803).

In the related-art method, an epilepsy seizure is detected based only ona change of the heart beat rate, and therefore there is a possibilitythat an event which is not an epilepsy seizure may be erroneouslydetected as an epilepsy seizure. Moreover, there is a furtherpossibility that the sensitivity of detection of an epilepsy seizurecannot be improved.

SUMMARY

This invention provides an apparatus and method for detecting anepilepsy seizure which detects the epilepsy seizure based on a reductionof the heart beat interval and a change of the fluctuation level of theheart beat interval, thereby enabling an epilepsy seizure to be detectedwithout erroneous detection and highly sensitively.

An aspect of the invention provides an apparatus for detecting anepilepsy seizure, the apparatus comprising: a graph generating unitconfigured to generate an orthogonal graph where values of heart beatintervals are sequentially plotted while a heart beat interval at anarbitrary timing is set as a first coordinate and a next heart beatinterval is set as a second coordinate; and a seizure detecting unitconfigured to detect the epilepsy seizure based on: a change of adistribution of a group of the plotted values in a first directionperpendicular to a straight line passing an origin and a point where thefirst and second coordinates are equal to each other; and a change of adistribution of the group of the plotted values in a second directionparallel to the straight line.

The seizure detecting unit may detect the epilepsy seizure based on: achange of a standard deviation of the distribution in the firstdirection; and a change of a standard deviation of the distribution inthe second direction.

The seizure detecting unit may detect the epilepsy seizure based on achange of a ratio of the standard deviation of the distribution in thefirst direction to the standard deviation of the distribution in thesecond direction.

The seizure detecting unit may detect the epilepsy seizure based on asituation where the ratio is equal to or smaller than a presetthreshold.

The threshold may be set for each subject of the heart beat intervals.

The apparatus may further comprise a detection signal outputting unitconfigured to, when the seizure detecting unit detects the epilepsyseizure, output a detection signal.

The apparatus may further comprise a heart beat interval measuring unitconfigured to measure the heart beat intervals in continuous heartbeats. The graph generating unit may plot the values of the heart beatintervals which are measured by the heart beat interval measuring unit.

The orthogonal graph may be a two-dimensional orthogonal graph, thefirst coordinate may be a lateral coordinate, and the second coordinatemay be a vertical coordinate.

An aspect of the invention provides a method of detecting an epilepsyseizure, the method comprising: generating an orthogonal graph wherevalues of heart beat intervals are sequentially plotted while aheartbeat interval at an arbitrary timing is set as a first coordinateand a next heart beat interval is set as a second coordinate; anddetecting the epilepsy seizure based on: a change of a distribution of agroup of the plotted values in a first direction perpendicular to astraight line passing an origin and a point where the first and secondcoordinates are equal to each other; and a change of a distribution ofthe group of the plotted values in a second direction parallel to thestraight line.

The epilepsy seizure may be detected based on: a change of a standarddeviation of the distribution in the first direction; and a change of astandard deviation of the distribution in the second direction.

The epilepsy seizure may be detected based on a change of a ratio of thestandard deviation of the distribution in the first direction to thestandard deviation of the distribution in the second direction.

The epilepsy seizure may be detected based on a situation where theratio is equal to or smaller than a preset threshold.

The threshold may be set for each subject of the heart beat intervals.

The method may further comprise: outputting a detection signal when theepilepsy seizure is detected.

The method may further comprise: measuring the heart beat intervals incontinuous heart beats.

The orthogonal graph may be a two-dimensional orthogonal graph, thefirst coordinate may be a lateral coordinate, and the second coordinatemay be a vertical coordinate.

An aspect of the invention provides a non-transitory computer-readablerecording medium in which a computer program causing a computer toexecute the method is recorded.

An aspect of the invention provides an apparatus for detecting anepilepsy seizure, the apparatus comprising: a graph generating unitconfigured to generate an orthogonal graph where values of heart beatintervals are sequentially plotted while a heart beat interval at anarbitrary timing is set as a first coordinate and a next heart beatinterval is set as a second coordinate; and a seizure detecting unitconfigured to detect the epilepsy seizure based on a change of atemporal locus of the plotted values in the graph.

An aspect of the invention provides a method of detecting an epilepsyseizure, the method comprising: generating an orthogonal graph wherevalues of heart beat intervals are sequentially plotted while a heartbeat interval at an arbitrary timing is set as a first coordinate and anext heart beat interval is set as a second coordinate; and detectingthe epilepsy seizure based on a change of a temporal locus of theplotted values in the graph.

An aspect of the invention provides a non-transitory computer-readablerecording medium in which a computer program causing a computer toexecute the method according is recorded.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an apparatus for detecting an epilepsyseizure according to an embodiment of the invention.

FIG. 2 is a view illustrating a graph in which Poincaré plots areplotted before and after an occurrence of an epilepsy seizure.

FIG. 3 is a view illustrating parameters which are used in detection ofan epilepsy seizure.

FIG. 4 is a view illustrating a flowchart of a program (method) fordetecting an epilepsy seizure according to the embodiment of theinvention.

FIG. 5 is a view illustrating a graph in which Poincaré plots areplotted before and after an occurrence of an epilepsy seizure.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, an apparatus and program (method) for detecting an epilepsyseizure according to an embodiment of the invention will be described indetail with reference to the drawings.

FIG. 1 is a block diagram of the apparatus 1 for detecting an epilepsyseizure according to the embodiment of the invention.

As shown in FIG. 1, the apparatus 1 for detecting an epilepsy seizureaccording to the embodiment includes a biological signal amplifier 100and an information processor 200.

The biological signal amplifier 100 is a unit which outputs a temporalvariability of the action potential of cardiomyocyte cells due to theheart beat of the subject, in the form of an electrocardiographicsignal. The biological signal amplifier 100 may be configured, forexample, by: electrodes which are attached to the chest or four limbs ofthe subject, and which pick up the electrocardiographic signal; anamplifier which amplifies the electrocardiographic signal; a filterwhich removes noises from the electrocardiographic signal amplified bythe amplifier; an A/D converter which digitizes the electrocardiographicsignal which is amplified, and from which noises are removed; and anexternal interface which is used for bidirectionally communicating withan external device.

The information processor 200 includes a controller 201, a data storage202, a display 203, a communication interface 204, a heartbeat intervaldetector 205, a graph generator 206, a seizure detector 207, and aninputting unit 208. The information processor 200 may be configured, forexample, by a PC (Personal Computer).

The controller 201 controls the components of the information processor200, and performs various calculation processes in accordance withprograms.

The data storage 202 stores electrocardiogram data which are receivedfrom the biological signal amplifier 100, various other data, andvarious programs.

The display 203 can display an electrocardiogram based on theelectrocardiogram data which are received from the biological signalamplifier 100. Furthermore, the display 203 can display a graph ofPoincaré plots generated by the graph generator 206.

Poincaré plots are plots in which, in continuous heart beats, heart beatintervals are sequentially plotted in a two-dimensional orthogonal graphwhile the heart beat interval RR_(n) at an arbitrary timing is set as alateral coordinate, and the next heartbeat interval RR_(n+1) is set as avertical coordinate. The heart beat interval RR means the timedifference between the peak of the R wave at an arbitrary timing in anelectrocardiogram, and that of the next R wave. The suffix n of theheartbeat interval RR_(n) means the order of the heartbeat interval RR,and the heart beat interval RR_(n+1) indicates the next heart beatinterval subsequent to the heart beat interval RR_(n).

The communication interface 204 receives the electrocardiogram data fromthe biological signal amplifier 100, and bidirectionally communicateswith the biological signal amplifier 100 to mutually transmit andreceive various kinds of information.

The heart beat interval detector 205 detects the heart beat interval RRbased on the electrocardiogram data received from the biological signalamplifier 100. The heart beat interval RR can be detected by storing theelectrocardiogram waveform in the data storage 202, detecting peaks of Rwaves which are adjacent to each other in the stored electrocardiogramwaveform, by calculation, and calculating the interval between the peaksof the R waves.

The graph generator 206 generates a graph of Poincaré plots, based onthe heart beat interval RR which is continuously detected for each beatby the heart beat interval detector 205. Namely, the graph generator 206sequentially plots heart beat intervals in continuous heart beats, in atwo-dimensional orthogonal graph while the heart beat interval RR_(n) atan arbitrary timing is set as a lateral coordinate, and the next heartbeat interval RR_(n+1) is set as a vertical coordinate.

The seizure detector 207 detects an epilepsy seizure based on the graphof Poincaré plots generated by the graph generator 206. The method ofdetecting an epilepsy seizure by the seizure detector 207 will bedescribed later.

When an epilepsy seizure is detected, the seizure detector 207 mayoutput a detection signal through the communication interface 204. Inthis case, the detection signal can be used as a trigger for initiatingmeans for avoiding a danger associated with an epilepsy seizure.

The inputting unit 208 is a unit to which information for identifyingthe subject is to be input. The information for identifying the subjectwhich is input through the inputting unit 208 is stored in the datastorage 202 while being correlated with the electrocardiogram data ofthe subject and the graph of Poincaré plots.

FIG. 2 is a view illustrating a graph in which Poincaré plots aregenerated before and after an occurrence of an epilepsy seizure. In thegraph shown in FIG. 2, the abscissa indicates the heart beat intervalRR_(n) which is obtained from the electrocardiogram waveform, and theordinate indicates the next heart beat interval RR_(n+1) which isobtained from the electrocardiogram waveform, and which is temporallyadjacent to the heart beat interval RR_(n).

The Poincaré plots enclosed by the broken line P in FIG. 2 are Poincaréplots before the occurrence of an epilepsy seizure, and those enclosedby the broken line I are Poincaré plots after the occurrence of theepilepsy seizure.

As seen from the graph of FIG. 2, when an epilepsy seizure occurs, theheart beat interval RR is rapidly shortened, and the fluctuation whichis the width of distribution of Poincaré plots in the directionperpendicular to the straight line RR_(n+1)=RR_(n) is decreased. Namely,when an epilepsy seizure does not occur, Poincaré plots form a loosecluster in the upper right side of the graph of FIG. 2, and, when anepilepsy seizure occurs, rapidly move to the lower left side to convergeinto the vicinity of the straight line RR_(n+1)=RR_(n). In FIG. 2, thestraight line RR_(n+1)=RR_(n) is shown in the graph, but the straightline may not be shown in the graph.

In the embodiment, an epilepsy seizure is detected based on twophenomena caused by an occurrence of an epilepsy seizure, i.e., aphenomenon in which the heart beat interval RR is shortened, and that inwhich the fluctuation of Poincaré plots is decreased. When an epilepsyseizure is detected by using the two characteristic phenomena, anepilepsy seizure can be detected without erroneous detection and highlysensitively.

FIG. 3 is a view illustrating parameters which are used in detection ofan epilepsy seizure by the seizure detector 207. In the graph shown inFIG. 3, similarly with FIG. 2, the abscissa indicates the heart beatinterval RR_(n) which is obtained from the electrocardiogram waveform,and the ordinate indicates the next heart beat interval RR_(n+1) whichis obtained from the electrocardiogram waveform, and which is temporallyadjacent to the heart beat interval RR_(n).

The dots shown in FIG. 3 are Poincaré plots, and the straight lineRR_(n+1)=RR_(n) is also shown. The arrow Vd shown in FIG. 3 indicatesthe direction perpendicular to the straight line RR_(n+1)=RR_(n), andSD1 which is indicated by the thick line on the arrow Vd indicates thestandard deviation of the distribution of Poincaré plots in thedirection of the arrow Vd. The arrow Pd shown in FIG. 3 indicates thedirection parallel to the straight line RR_(n+1)=RR_(n), and SD2 whichis indicated by the thick line on the arrow Pd indicates the standarddeviation of the distribution of Poincaré plots in the direction of thearrow Pd.

The seizure detector 207 detects an epilepsy seizure based on thestandard deviation SD1 (hereinafter, referred to simply as “SD1 value”)of the distribution of Poincaré plots in the direction Vd perpendicularto the straight line RR_(n+1)=RR_(n), and the standard deviation SD2(hereinafter, referred to simply as “SD2 value”) of the distribution ofPoincaré plots in the direction Pd parallel to the straight lineRR_(n+1)=RR_(n). Specifically, the seizure detector 207 detects anepilepsy seizure based on a change of a value (hereinafter, referred tomerely as “SD1/SD2 ratio”) which is obtained by dividing the SD1 valueby the SD2 value. The seizure detector 207 compares the presentlycalculated SD1/SD2 ratio with the previously calculated SD1/SD2 ratio,and, if it is determined that there is a significant difference, candetect an epilepsy seizure.

When an epilepsy seizure is detected based on a change of the SD1/SD2ratio as described above, the rate of erroneous detection of an epilepsyseizure can be further lowered, and the detection sensitivity can bemore improved.

Preferably, a threshold for detecting an epilepsy seizure from a changeof the SD1/SD2 ratio is set for each subject. This can further improvethe sensitivity of detection of an epilepsy seizure.

An adequate weight may be applied to the SD1 value and SD2 value whichprovide the SD1/SD2 ratio.

Alternatively, the seizure detector 207 detects an epilepsy seizurebased on a situation where the SD1 value is equal to or smaller than apreset threshold, and the SD2 value is equal to or larger than a presetthreshold. Namely, individual thresholds may be set for the SD1 valueand the SD2 value, respectively, and, when the both values exceed thethresholds, an epilepsy seizure may be detected.

Alternatively, an epilepsy seizure may be detected based on a change ofa value which is obtained by dividing the SD2 value by the SD1 value.

FIG. 4 is a view illustrating a flowchart of a program (method) fordetecting an epilepsy seizure according to the embodiment. The program(method) for detecting an epilepsy seizure can be executed by theinformation processor 200.

The controller 201 receives electrocardiogram data from the biologicalsignal amplifier 100 through the communication interface 204 (S401). Thereceived electrocardiogram data are stored in the data storage 202.

The graph generator 206 generates a graph of Poincaré plots by using theheart beat interval RR which is detected by the heart beat intervaldetector 205 based on the electrocardiogram data stored in the datastorage 202 (S402).

The seizure detector 207 calculates the SD1/SD2 ratio based on Poincaréplots generated by the graph generator 206 (S403). The previouslycalculated SD1/SD2 ratio is compared with the presently calculatedSD1/SD2 ratio, and it is determined whether there is a significantdifference or not (S404).

If, in the comparison between the previously calculated SD1/SD2 ratiowith the presently calculated SD1/SD2 ratio, it is determined that thereis no significant difference, steps S401 to S403 are repeated (S404:NO).

If, in the comparison between the previously calculated SD1/SD2 ratiowith the presently calculated SD1/SD2 ratio, it is determined that thereis a significant difference, the seizure detector 207 outputs thedetection signal through the communication interface 204 (S405).

Steps S401 to S405 are repeated until the user inputs instructions forterminating the detection of an epilepsy seizure (S406).

An epilepsy seizure has a feature that it has unpredictability andrecurrence. When a seizure accompanied by impairment of consciousnessoccurs, therefore, the patient cannot perform a voluntary motion, andhence there is a risk that not only the patient but also another personmay cause an accident. Because an epilepsy seizure has theabove-described feature, the risk cannot be always reduced unless aseizure can be detected.

When the apparatus for detecting an epilepsy seizure according to theembodiment detects the epilepsy seizure and outputs the detectionsignal, however, the means for avoiding a danger associated with anepilepsy seizure can be initiated by using the detection signal as atrigger. For example, it is contemplated that the occurrence is informedto the third party, or that, in the case where the subject operates amachine, a function of stopping the machine is activated.

When the apparatus for detecting an epilepsy seizure according to theembodiment detects the epilepsy seizure and outputs the detectionsignal, the detection signal can be used as a trigger for activating atreatment apparatus for suppressing a seizure. For example, thedetection signal for an epilepsy seizure may be used as a trigger foractivating a chemical injection pump for suppressing a seizure, or atreatment apparatus such as a brain/neural stimulator.

EXAMPLE

An example of the embodiment will be described.

(Conditions and Method)

(a) Subjects: 19 persons (15 males) of ages 19 to 67.

(b) With respect to 19 epilepsy seizures of refractory temporal lobeepilepsy, the heart beat interval RR was measured by usingelectrocardiograms generated by an electrocardiograph having a samplingfrequency of 200 Hz or 500 Hz. Poincaré plots were generated, and theSD1/SD2 ratios were calculated.

(c) It was considered whether differences of averages of SD1 value, SD2value, and the SD1/SD2 ratio before and after a seizure are significantor not.

The p-value shown in Table 1 below indicates the probability that, inthe case where a null hypothesis that there is no difference in SD1value, SD2 value, and the SD1/SD2 ratio before and after an occurrenceof an epilepsy seizure is generated, measured data are incidentallyconsistent with the null hypothesis.

(Results)

Table 1 below shows the SD1 value, SD2 value, and SD1/SD2 ratio whichwere measured in the example, and the p-value.

TABLE 1 Before epilepsy After epilepsy seizure seizure p-value SD1 value[ms] 33.3 ± 30.2 11.0 ± 4.3  0.004 SD2 value [ms] 74.4 ± 49.0 117.1 ±50.6  0.012 SD1/SD2 ratio 0.46 ± 0.27 0.11 ± 0.06 <0.001

As shown in Table 1, it has been proven that the difference between theSD1/SD2 ratios before and after an occurrence of an epilepsy seizure issignificant, and an epilepsy seizure can be sufficiently detected basedon a change of the SD1/SD2 ratio.

In the above, the apparatus and program (method) for detecting anepilepsy seizure according to the embodiment of the invention have beendescribed. The embodiment attains the following effects.

An epilepsy seizure is detected based on a change of the heart beatinterval and that of the fluctuation level of the heart beat interval,thereby enabling an epilepsy seizure to be detected without erroneousdetection and highly sensitively.

As shown in FIG. 5, moreover, a change of the heart beat interval isconsidered as a movement of a temporal locus of a dot group in Poincaréplots. Next, the fluctuation of the heart beat interval for each heartbeat is measured by a change of the distance between the straight linepassing the origin and a point where the lateral and verticalcoordinates are equal to each other, and a plotted dot. Therefore, anepilepsy seizure can be detected more objectively, without erroneousdetection, and highly sensitively. However, the display of astraight-line graph is not an essential condition.

Furthermore, an epilepsy seizure is detected based on a change of theratio of the standard deviation of the distribution in a graph ofPoincaré plots in the direction perpendicular to the straight linepassing the origin and a point where the lateral and verticalcoordinates are equal to each other, to that of the distribution in thedirection parallel to the straight line. Therefore, an epilepsy seizurecan be detected with higher reliability.

Furthermore, an epilepsy seizure is detected based on a situation wherethe ratio of the standard deviation of the distribution in a graph ofPoincaré plots in the direction perpendicular to the straight linepassing the origin and a point where the lateral and verticalcoordinates are equal to each other, to that of the distribution in thedirection parallel to the straight line is equal to or smaller than athreshold. Therefore, an epilepsy seizure can be detected with furtherhigher reliability.

When the threshold is set for each subject, furthermore, the sensitivityof detection of an epilepsy seizure can be further improved.

When an epilepsy seizure is detected, the detection signal may beoutput, so that the detection signal can be used as trigger for stoppinga machine which is operated by the patient. Therefore, it is possible toavoid a danger associated with an epilepsy seizure.

The apparatus and program (method) for detecting an epilepsy seizureaccording to the invention are not limited to the above-describedembodiment.

In the embodiment, for example, it has been described that the apparatusfor detecting an epilepsy seizure is configured by an informationprocessor and programs which are to be executed by the processor.Alternatively, apart or all of the programs may be replaced withhardware such as a custom LSI (Large Scale Integration).

For example, heart beat intervals in continuous heart beats may bemeasured, and the heart beat interval at an arbitrary timing and thenext heart beat interval may be sequentially plotted on the YZ-, XZ-, orXY-axis of a three-dimensional orthogonal graph.

According to an aspect of the invention, since an epilepsy seizure isdetected based on a reduction of the heart beat interval and a change ofthe fluctuation level of the heart beat interval, an epilepsy seizurecan be detected without erroneous detection and highly sensitively.

What is claimed is:
 1. An apparatus for detecting an epilepsy seizure,the apparatus comprising: a graph generating unit configured to generatean orthogonal graph where values of heart beat intervals aresequentially plotted while a heart beat interval at an arbitrary timingis set as a first coordinate and a next heart beat interval is set as asecond coordinate; and a seizure detecting unit configured to detect theepilepsy seizure based on: a change of a distribution of a group of theplotted values in a first direction perpendicular to a straight linepassing an origin and a point where the first and second coordinates areequal to each other; and a change of a distribution of the group of theplotted values in a second direction parallel to the straight line,wherein the seizure detecting unit detects the epilepsy seizure basedon: a change of a standard deviation of the distribution in the firstdirection; and a change of a standard deviation of the distribution inthe second direction, and wherein the seizure detecting unit detects theepilepsy seizure based on a change of a ratio of the standard deviationof the distribution in the first direction to the standard deviation ofthe distribution in the second direction.
 2. The apparatus according toclaim 1, wherein the seizure detecting unit detects the epilepsy seizurebased on a situation where the ratio is equal to or smaller than apreset threshold.
 3. The apparatus according to claim 2, wherein thethreshold is set for each subject of the heart beat intervals.
 4. Theapparatus according to claim 1, further comprising a detection signaloutputting unit configured to, when the seizure detecting unit detectsthe epilepsy seizure, output a detection signal.
 5. The apparatusaccording to claim 1, further comprising a heart beat interval measuringunit configured to measure the heart beat intervals in continuous heartbeats, wherein the graph generating unit plots the values of the heartbeat intervals which are measured by the heart beat interval measuringunit.
 6. The apparatus according to claim 1, wherein the orthogonalgraph is a two-dimensional orthogonal graph, the first coordinate is alateral coordinate, and the second coordinate is a vertical coordinate.7. A method of detecting an epilepsy seizure, the method comprising:generating an orthogonal graph where values of heart beat intervals aresequentially plotted while a heart beat interval at an arbitrary timingis set as a first coordinate and a next heart beat interval is set as asecond coordinate; and detecting the epilepsy seizure based on: a changeof a distribution of a group of the plotted values in a first directionperpendicular to a straight line passing an origin and a point where thefirst and second coordinates are equal to each other; and a change of adistribution of the group of the plotted values in a second directionparallel to the straight line, wherein the epilepsy seizure is detectedbased on: a change of a standard deviation of the distribution in thefirst direction; and a change of a standard deviation of thedistribution in the second direction, and wherein the epilepsy seizureis detected based on a change of a ratio of the standard deviation ofthe distribution in the first direction to the standard deviation of thedistribution in the second direction.
 8. The method according to claim7, wherein the epilepsy seizure is detected based on a situation wherethe ratio is equal to or smaller than a preset threshold.
 9. The methodaccording to claim 8, wherein the threshold is set for each subject ofthe heart beat intervals.
 10. The method according to claim 7, furthercomprising: outputting a detection signal when the epilepsy seizure isdetected.
 11. The method according to claim 7, further comprising:measuring the heart beat intervals in continuous heart beats.
 12. Themethod according to claim 7, wherein the orthogonal graph is atwo-dimensional orthogonal graph, the first coordinate is a lateralcoordinate, and the second coordinate is a vertical coordinate.
 13. Anon-transitory computer-readable recording medium in which a computerprogram causing a computer to execute the method according to claim 7 isrecorded.